JPH03503896A - How to inhibit polymerization of acid monomers - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

[Detailed description of the invention] Monomer person, method Hikari”L Scream J1 The present invention relates to a composition for inhibiting the polymerization of acrylic acid and a method for producing this composition. . The present invention is particularly useful in preventing polymerization during distillation of acrylic acid. Therefore The invention also relates to a method for stabilizing the distillation of acrylic acid.

Acrylic acid was first produced in 1847, and its polymerization took place in the first generation. The technology involved in the production and polymerization of acrylic acid has been known for more than centuries. Measures to address these difficulties have only begun to emerge since the 1930s. Acrylic The process of producing acid, particularly its concentration and purification, usually involves various thermal distillation techniques.

The major part of the loss of acrylic acid during these operations is due to thermal polymerization of acrylic acid. It is widely known that However, in these distillation operations, thermal Since energy is essential, there are various methods that inhibit polymerization despite the presence of heat. Various devices or means have been considered over the years. Especially under mild operating conditions. In order to avoid the residence time of acrylic acid, rapid processing and polymerization inhibition in the distillation equipment The provision of inhibitors has been employed for this purpose.

Up to now, a large number of polymerization inhibitors have been tried alone or in combination of two or more, and In the process of vapor distillation, molecular oxygen is often bubbled into the distillation column to create a synergistic effect. Ta. Such inhibitors include paramethoxyphenol (herein referred to as hydrochloride). quinone monomethyl ether (MEHQ)), phenothiazine (PTZ) , catechol, methylene blue, diphenylamine, various organic copper salts, especially aromatic Group nitrone compounds and other phenol, amine, nitro and quinone compounds and their inorganic salts. However, in most cases these polymerization inhibitors It was particularly effective in inhibiting either gas phase polymerization or liquid phase polymerization.

U.S. Pat. No. 3,674,651 describes molecular oxygen, diphenylamine or its derivatives. Polymerization inhibitor composition for acrylic acid, comprising ester and benzoquinone or MEHQ is disclosed. However, the method for achieving synergy with this composition is This inhibitor is introduced partially into the gas phase, while an additional common liquid phase polymerization inhibitor is added to the liquid phase. However, it may be necessary to supply air or oxygen upward from the bottom of the column for stabilization. be.

U.S. Pat. Disclosed is a method for treating copper salts of certain carbamic acids or salicylic acids. at least one inhibitor such as MEHQ and PTZ in combination with a bubble in the distillation column. Requires standing air or molecular oxygen.

In 1976, Jursich in US Patent No. 3.959.358 Phenols for acrylic acid esters such as HQ (baramethoxyphenol) The excess molar amount of the polymerization inhibitor composition is combined with various amine inhibitors such as PTZ. It was argued that this would be effective in the presence of dissolved oxygen. He says that phenolic inhibitors are acids It has been argued that heat-activated polymerization is well prevented when little or no element is present. . However, according to Jursich, oxygen in the ester solution, e.g. Phenolic inhibitors alone are not suitable if they are present due to polymerization initiators such as It is. Therefore, to capture dissolved oxygen without bubbling air or oxygen into the column. Various amine inhibitors, especially phenothiazine (PTZ), have been used. but , unlike acrylic esters, this theory is not valid for acrylic acid. I found out.

For many years, N-nitrosophenylhydroxylamine (NPH) has been used as a monomer It was known as a stabilizer that inhibits chemical reactions. For example, U.S. Patent No. 2.758.131 See also German Patent No. 1.180.733. Also, U.S. Patent No. 3, No. 426.063 describes polymerization of water-soluble acrylic esters that is not polymerization of acrylic acid. It has been disclosed that NPH is used to inhibit Previously, polymerization inhibition for acrylic acid The use of NPH as a detergent has a high level of dissolved residue that adversely affects distillation operations. There were a number of drawbacks, including the need to use concentrations.

According to the disclosure of St. Ewart in U.S. Pat. No. 4.210.493, Certain anaerobic conditions common to the distillation of acrylic acid, e.g. gas phase space, top of the column, gas-free venting. Prohibit polymerization of acrylic acid in reflux and especially when vacuum distillation is involved. The inhibiting properties of most known inhibitors are due to the lack of oxygen flowing through the device. Completely lost. Certain aliphatic C-nitroso compounds are present in the absence of air or oxygen. It is said that acrylic acid overcomes this problem when inhibiting gas phase polymerization.

U.S. Pat. No. 4,310,676 discloses that acrylic acid is Another polymerization inhibitor containing phenothiazine (PTZ) and varanitrosophenol A stopper composition is disclosed.

A method that inhibits the polymerization of acrylic acid in both gas and liquid phases without the problems of conventional technology. New and improved compositions would represent a substantial advance in the art.

Hikari L Summer JJ11 The main objective of the present invention is to provide a synergistic composition that inhibits the polymerization of acrylic acid. .

The six important objectives are to achieve exceptional performance in both the liquid and gas phases during distillation. An object of the present invention is to provide an acrylic acid polymerization inhibiting composition having the following properties.　　　　.

Still another object is to provide a method for producing a synergistic polymerization inhibiting composition of acrylic acid. be.

Another object of the present invention is to improve the performance of NPH as a polymerization inhibitor for acrylic acid. It is to provide law.

Yet another object of the present invention is to improve the performance of MEHQ as a polymerization inhibitor for acrylic acid. The aim is to provide new ways to improve

Yet another main objective of the present invention is that there is no need to bubble air or molecular oxygen into the distillation apparatus. Another object of the present invention is to provide a method for stabilizing the distillation of acrylic acid.

An additional object of the invention is that there is no need to bubble air or oxygen during distillation or purification. An object of the present invention is to provide a novel method for imparting a synergistic effect to polymerization-inhibiting compositions of acrylic acid. .

Further objects and advantages of the present invention are the following detailed description and implementation of preferred B. It will be clear to those skilled in the art by reference to the examples.

、　Na-・　Bi゛　E According to the present invention, the thermal polymerization of acrylic acid in the liquid and/or gas phase is performed with pre-oxygen activation. N-nitrosophenylhydroxylamine (NPH) and paramethoxyphenol (referred to herein as hydroquinone monomethyl ether (MEHQ)). It has been found that this can be very effectively inhibited by a composition comprising:

Obtained by combining NPH and MEHQ to obtain the composition of the present invention Effective inhibition of polymerization is achieved by using either inhibitor alone or with others such as phenothiazines. compared to the polymerization inhibition obtained when used in combination with commercially available inhibitors of It's much bigger.

Chemically, the interaction between NPH and oxygen and the interaction between NPH and MEHQ are as follows: Although unclear, a detailed explanation of the invention is possible. NPH and MEHQ are a normal group. It was found that the combination did not provide a very good polymerization inhibiting effect. NPH is MEH A form that reacts or interacts with Q very quickly and has no effect, i.e. a form or reaction. will form a product. However, NPH is well-stirred and is often used here. It is activated by dissolved oxygen in an acid solution that has been left standing for a short period of time, often called the residence time. or interact with dissolved oxygen in some way or be oxidized by dissolved oxygen. cormorant. Even if this reaction or interaction with oxygen in NPH occurs, NPH and MEHQ slow compared to interactions with Therefore, the amount of MEHQ that can be present within the residence time as long as the amount of NPH is in substantial molar excess than the number of moles of NPH. When MEHQ is added in molar amounts, a very effective polymerization inhibitor is obtained. did Therefore, when combined with MEHQ, it becomes very effective as a polymerization inhibitor for acrylic acid. Although the exact chemical species of NPH is not known, this ingredient in the compositions of the present invention will be used hereafter. It is written as oxygen active NPH.

Preoxygen active NPH is NPH, preferably ammonia, often called cuperone. Derived from an aqueous solution of um salt. What can be used to prepare such aqueous solutions? Other water-soluble salts of NPH that can be formed include alkali metal salts such as sodium salts of the genus; salts of amines such as ethylamine; salts of alkanoids such as ethanolamine salts of amines; and water, alcohols, esters, ethers, others, acrylics At least 5% soluble in a solvent that is compatible with the acid and does not adversely affect the properties of the monomer. Examples include salts of NPH with solubility. Solutions of convenient concentrations can be used. Ru. When using cuperone as the NPH source, use an aqueous solution of approximately 10% NPH. was particularly desirable and suitable. Final monomer to stabilize against thermal polymerization If the molar amount of NPH is small enough to provide the desired amount or concentration of NPH in the of weak acids such as acrylic acid and acetic acid and stirred to form a pre-acid of NPH. The residence time required for elemental activation is provided. Desirable N for effective polymerization inhibitor compositions The amount of PH0 is 50 pp+* to 150 ppm+ based on the final weight of acid monomer. A range of is preferred. Therefore, 0.001 g per 1 d of acrylic acid (1 g/ Preferably, between 0.006 g and 6 g/l of NPH is used, i.e. Pre-oxygen stage of this method for preparing what is referred to herein as pre-oxygen active NPH. The NPH between floors is preferably in an amount of 2000 ppm.

Acrylic acid is approximately 100 to 200 ppm MEHQ as noted in the preliminary control. , pre-stabilized by this amount of MEHQ alone or in combination with other inhibitors such as PTZ. The modified acrylic acid has inferior performance compared to the composition of the present invention. Also, this concentration of M Acrylic acid prestabilized with EHQ usually becomes less effective with the addition of NPH. . Nevertheless, there is a substantial excess of NPH over MEHQ in this acid monomer. present in the solution and provide sufficient residence time after mixing the NPH into the solution. It has been found that bright pre-oxygen active NPH can be produced. A preferred embodiment of the present invention According to MEHQ1, a small amount of acrylic acid containing 0 to 200 ppm MEHQ was added to MEHQ1. Mixed with a substantial molar excess of NPH ranging from NPHIO to 50 moles per mole It is necessary to. However, oxygen activation occurs even in the complete absence of MEHQ. This can also occur when NPH is added to weak acids other than acrylic acid, such as acetic acid. What you get should be noted.

Following the preparation of pre-oxygenated NPH, MEHQ is added. Preferably, M.E. HQ is dissolved in acrylic acid in a stabilizing amount, and this MEHQ solution is Based on the weight of 50 to 150 ppn+ of NPH and acrylic acid. Pre-oxygen activation to give a composition containing 200-400 ppm MEHQ. added to sexual NPH.

The inhibitor compositions of the present invention can be purified during the production of acrylic acid and by distillation of this monomer. It can be used to inhibit polymerization during manufacturing. This inhibitor composition is prepared in advance or may come into contact with impure assembly knobs and It is possible to generate earthenware with . Mixing a 7-year-old with an inhibitor composition of the present invention The product can be distilled under substantially anaerobic conditions and condensed outside the distillation zone. child Inhibitors are also added to the distillation column shelves during distillation to prevent the formation of polymers there. You can also do that. Furthermore, purified monomers can be purified under anaerobic conditions by addition of the inhibitor of the present invention. It is effectively stabilized against polymerization during storage at temperatures above 15°C.

The compositions of the present invention provide synergistic polymerization inhibition without the need for bubbling oxygen into the distillation column. the required residence time necessary to provide a depressant effect and to generate preoxygen active NPH. are usually avoided during distillation, but a distillation column of sufficient length to achieve the objectives of the present invention. If using It would be possible to pre-oxygen activate a solution consisting of:

As mentioned above, the desired concentration of NPH and MEHQ is M for preoxygen active NPH. A preferred weight ratio of EHQ is in the range of 0.02 to about 0.05. Preferably 0.1 A ratio of 25 to 0.75 is used, more preferably a ratio of 0.4 to 0.6.

Typical distillation of acrylic acid is carried out at about 90°C under a pressure of about 50 mmHg to 150 mmHg. It is vacuum distilled at a temperature ranging from 120°C to 120°C. The composition of the present invention It serves to stabilize such distillations very effectively. Furthermore, the present invention It can be useful for the production of acrylic acid by the chemical method.

The polymerization inhibiting composition of the present invention is very advantageous in both the liquid and gas phases of monomer distillation. It shows a great effect. Initiation of polymerization in the liquid phase confirms the effectiveness of the polymerization inhibitor composition This is manifested by the occurrence of turbidity, which is measured during the test. The effectiveness of inhibition in the gas phase is Positive or negative weight gain for bob cone polymer beads placed at the top of the distillation column Measured using traditional methods.

The present invention will be explained in more detail with reference to the following examples.

-Back-" L-Yuichi In each example, a round bottom with a Vigreux column (30 CIR length) and a vacuum source. The effectiveness of the test inhibitor was determined using flasks.

The flask was heated with an oil bath and stirring was performed with a magnetic stirrer.

Accurately weighed styrene-butadiene rubber bob cone particles (0,01-0,0 Heat the contents of the flask in a small stainless steel mesh basket containing >2 g While the particles are suspended in the gas phase in the flask, the column is suspended sufficiently above the liquid phase. fixed to the top of the.

For each test, 100 i of glacial acrylic acid containing the test inhibitor was placed in a flask. Ta. The acrylic acid is stirred and brought under reduced pressure to reflux well above the Vigreux column. heated with. The reflux was carried out under anaerobic conditions at a pressure of 50 to 60 m of mercury, and the temperature used was The temperature was 95-105°C. The duration of reflux is 6 hours or until polymerization occurs rapidly. If this was observed, the period of time was less than 6 hours.

Both gas phase and liquid phase polymerization inhibition were measured. Bobco at the end of the reflux time The grain-like particles were collected, dried, and weighed to calculate the percent weight change of the zero particles. It can be seen that gas phase polymerization was completely inhibited due to the absence of zero weight increase or negative weight change. The relative degree of gas phase polymerization is determined by the zero weight increase. If liquid phase polymerization is inhibited, the liquid phase becomes cloudy. This turbidity indicates the presence of polymer in the liquid. show.

Specific examples are shown below, and the results are shown in Table 1.

Example 1 0.013 g NP in 200 ppm MEHQ standardized acrylic acid 6d H was added as a 10% aqueous solution to give a substantial molar excess of NPH. Stir this solution. Stir and let stand for 3 to 4 minutes. It was then stabilized with 200 ppm MEHQ. Acrylic acid 94M1 was placed in the flask of the test apparatus and mixed with the aliquot 6- above. 100% solution containing 124pp-〇NPH and 200ppm MEHQ I got d. This solution was refluxed for 6 hours, and as shown in Table 1, an extraordinary polymerization inhibition was observed. Obtained. Aliquot 6d of acrylic acid containing a substantial molar excess of NPH was By stirring under ambient conditions, the mixture is brought into equilibrium with the air and after a residence time of 3 to 4 minutes. The time period was sufficient for oxygen activation of gNPH by dissolved oxygen. This allows MEHQ Synergistic inhibition of polymerization became possible when molar excess of was added.

Example 2 Add 0.0115 g of NPH to aliquot 5d of acrylic acid and let the solution cool for 4 minutes. Same procedure as Example 1 except that after standing for a while, it was mixed with aliquot 95d of acrylic acid. I followed the order. The final concentration of NPH tested was 110 pp-, Example As in case 1, an extraordinary inhibition of polymerization was observed.

Example 3 Acrylic acid stabilized with 200 ppm MEHQ is first distilled to essentially A similar procedure was followed as in Examples 1 and 2, except that acrylic acid without inhibitor was obtained. Ban Add 0.013 g of NPH as a 10% aqueous solution to 20 d of acrylic acid without agent, Mixed. 0.0227 g of this distilled inhibitor-free acrylic acid 80R1 of MEHQ was added and mixed. After 1 minute, mix the two solutions in the test flask. and tested for gas phase and liquid phase polymerization inhibition. The concentration of MEHQ is 216 ppm. The concentration of NPH was 124 ppm. The result is an extraordinary inhibition of polymerization. , and NPH activates oxygen without any need for MEHQ during the air equilibration time. Sexualized.

Example 4 The same procedures as in the previous examples were followed except that NPH was activated with oxygen in acetic acid.

Make a 3% aqueous solution (0.1 d) of 3 g of NPH in 4.5 d of acetic acid in a small bottle. The solution was mixed and allowed to stand for 1 minute. Next, this solution was added to 200 ppm of acrylic acid containing MEHQ and this solution was added to the flask of the test apparatus. In addition to this, polymerization inhibition was tested. Solutions tested are based on weight of acrylic acid with 29 ppm NPH and 200 ppr @ MEHQ based on the weight of acrylic acid. It contained. In this case too, polymerization inhibition was achieved in both the liquid and gas phases.

Example 5 Example except that 0.0018 g of NPH was added to acetic acid as a 1.8% aqueous solution. The same procedure as in 4 was followed. After about 1 minute, add 200 ppm (7) MEHQ to this solution. The mixture was tested by mixing it with 100 IIIt of acrylic acid. This composition All items are NPH17pp+i and MEHQ2 based on the weight of acrylic acid. It contained 00 ppm. Again, excellent results showed that the oxygen activation of NPH was performed in acetic acid. obtained if you go.

Example 6 Protect the first aliquot of NPH from air and prevent it from combining with oxygen for activation. The same procedure as in each of the above Examples was followed except that the procedure was different. MEHQ200ppm acrylic acid containing 6d was purged with nitrogen for several minutes to remove dissolved oxygen. Next Add 0.013 g of NPH to a 10% aqueous solution while maintaining a nitrogen gas seal over the solution. and mixed. NPs dissolved in acrylic acid while maintaining a nitrogen gas seal The H solution was allowed to stand for 4 minutes.The solution was then purified with MEH, also under a nitrogen gas blanket. Mixed with acrylic acid containing 200 ppm Q. Distill this mixture into a flask and tested for inhibition of polymerization. The amount of NPH is 124 ppm and the amount of MEHQ was 200 pp-. Polymerization inhibition results are extremely poor in both liquid and gas phases. It was.

This test showed that without oxygen activation, NPH in combination with MEHQ It has been demonstrated that the polymerization inhibition effect is inferior to that of acids.

Example 7 This example shows that oxygen activation of NPH is performed when MEHQ is present in molar excess. This embodiment differs from each of the embodiments described above in this respect. Acrylic acid containing MEHQ200ppn+ An additional 0.0253 g of MEHQ was added to the 5d container. This 5III 0.0123 g of NPH was added as a 10% aqueous solution to the 1 ml solution. This solution of 5- The molar ratio of MEHQ to NPH in the solution was 2.7. Let this solution stand for 6 minutes. After standing, it was mixed with 95-acrylic acid containing 200 ppm of MEHQ, and the final The 100d solution contains 117ppa+ of NPH and 440ppm of MEHQ. Ta.

This solution was tested for polymerization inhibition. The results obtained were very poor and this , when MEHQ is present in excess, NPH is not activated by exposure to dissolved oxygen. It is shown that.

Clearly, in acrylic acid, or even in other acids such as acetic acid, NPH is M Forms that react or interact with EHQ very quickly and do not provide good polymerization inhibition, i.e. See Table 1 for forming complexes or reaction products.

Example 8 Excess MEHQ was mixed with NPH as in Example 7.

However, no time was allowed for the NPH mixture to stand still. That is, NPH0 in the flask , 013 g was added with 100 d of acrylic acid containing 220 pp+s of MEHQ.

Therefore, the concentration of NPH is 124 ppm and the concentration of MEHQ is 200 ppm. It was pps+.

This solution was mixed and immediately transferred to a test apparatus and tested for polymerization inhibition. or Poor results were obtained and the interaction or reaction between MEHQ and NPH was very rapid. occur frequently, rendering the mixture ineffective and/or making oxygen activation of NPH impossible. That has been proven. This particular excess of MEHQ is not as large in the case of Example 7. won.

Example 9 Same as the example day except that the number of moles of NPH was substantially larger than the number of moles of MEHQ. A similar procedure was followed. That is, 100% of acrylic acid containing 200ppm of MEHQ. 0.065 g of NPH was added as a 10% aqueous solution to d under stirring, and NPH was 61 The amount was 9 ppm. The results were not very good, and the residence time increased the activation of NPH. and NPH is essential for acrylic acid if this oxygen activation is not performed. It was clearly shown that it is not a very good polymerization inhibitor.

Example 10 This example demonstrates the effectiveness of MEHQ alone as a polymerization inhibitor for acrylic acid under anaerobic conditions. This was done to measure gender. In other words, an aqueous solution that already contains 200 ppm of MEHQ. MEHQo, 0.012 g was added to 100 I11 of lylic acid. Therefore, this solution is MEH Q reached a concentration of 400 ppm and the solution was tested for polymerization inhibition. result shows that MEHQ is a much worse inhibitor than the composition of the present invention.

Example 11 This example shows that oxygen-activated NPH alone is effective in inhibiting the polymerization of acrylic acid. I went there to find out if it would work. Distilled acrylic acid 20d without other inhibitors 0.013 g of NPH was added as a 10% aqueous solution. Mix this solution under aerobic conditions. The mixture was allowed to stand for about 1 minute. Then add more 801d of distilled acrylic acid and add NPH Add 100 d of this solution with a concentration of 124 pp- to the test equipment to inhibit polymerization. was tested. However, because the acrylic acid polymerized very quickly, bob cone-like particles were formed. The basket containing the offspring is blown off its support and can be recovered from the polymerized acrylic acid. I couldn't come. This means that even when activated with oxygen, NPH alone is not effective for polymerization. It is not an inhibitor (it has been shown that it can even promote severe polymerization after 50 minutes of reflux). Ru.

Example 12 The purpose of doing this example is to use acrylic acid as an inhibitor in the presence of oxygen. Certain ineffective phenolic inhibitors, such as MEHQ, are called phenothiazines. It is effective by adding certain amine inhibitors that protect the oxygen By testing the premise of U.S. Pat. No. 3,959,358, which argued that Ru. As long as MEHQ is present in excess over phenothiazine (PTZ), the effective It is expected that there will be a ban on matching. However, in this example, it contains 200 ppm of MEHQ. acrylic acid was used with PTZ prepared by the same method that was effective against NPH. I processed it. That is, 0132 g of PTZO was added to 6 d of acrylic acid and mixed. PTZ was present in substantial molar excess. After leaving this 6-pot solution for a few minutes, apply Mixed with acrylic acid 941d. The concentration of PTZ is 126 ppm, and MEHQ is It was 200 ppm. As can be seen from Table 1, polymerization inhibition was very poor, and the present invention It was completely inferior to the composition.

Table 1 1 200 124 -2.4 6 hours 6 hours 2 200 110 -7.8 6 hours 6 hours 3 216 124 -1.2 6 hours 6 hours 4 200 29 -0.7 6 Time 6 hours 5 200 17 +2.7 6 hours 6 Time 6 200 124 639, 4 hours 1 hour 7 4 40 117 592, 5 hours 40 minutes 8 200 12 4 644, 5 hours 30 minutes 9 200 619 252, 70 minutes 45 minutes 10 400 277, 35 minutes 15 minutes 11 124 -” 50 minutes 20 minutes 12 200 636 6 hours 36 minutes International search report international search report

Claims (15)

[Claims] 1. a) Pre-oxygen active N-nitrosophenylhydroxylamine and b) Compositions comprising a molar excess of hydroquinone monomethyl ether. 2. N-nitrosophenyl hydroxy to hydroquinone monomethyl ether The set according to claim 1, wherein the relative weight ratio of the amines is from 0.02 to 0.95. A product. 3. 3. The composition of claim 2 further comprising acrylic acid. 4. 1. A method for producing a polymerization inhibitor composition, comprising: a) any hydroquinone moiety that may be present; N-nitrosophenylhydroxy in a substantial molar excess over the methyl ether a step of activating ruamine with oxygen; and b) in molar excess over the oxygen-active N-nitrosophenylhydroxylamine Step of adding sufficient amount of hydroquinone monomethyl ether to method including. 5. About 2000 ppm N-nitrogen in acrylic acid with the oxygen activation balanced with air. Concentrate the trosophenylhydroxylamine for about 1-4 minutes. 5. The method of claim 4, wherein the method is carried out in situ by ing). 6. The hydroquinone monomethyl ether added in step (b) is an acrylic acid solution. 5. The method of claim 4, added as: 7. A method for inhibiting the polymerization of acrylic acid, the method comprising: a) pre-oxygen activated N- Nitrosophenylhydroxylamiso and b) Hydroquinone monomethyl ether A method including processing with. 8. Hydroquinone monomethyl ether is a pre-oxygen active N-nitrosophenyl hydride. 8. The method of claim 7, wherein the amount is in molar excess relative to roxylamine. 9. The concentration of (a) is 10 to 400 ppm based on the weight of acrylic acid, and (b) Claim 7, wherein the concentration of is 100 to 500 ppm based on the weight of acrylic acid. Method described. 10. Claims wherein the weight ratio of (a) to (b) is from about 0.02 to about 0.95. The method described in scope item 7. 11. Add acrylic acid in balance with air to any hydroquinone monomethyl ether that may be present. N-nitrosophenylhydroxylamine in a substantial molar excess over and adding more hydroquinone monomethyl ether after an effective residence time. 8. A method according to claim 7, wherein (a) is prepared by: 12. 8. The method of claim 7, further comprising distilling the acrylic acid. . 13. Acrylic acid, pre-oxygen activated N-nitrosophenylhydroxylamine and A stabilized acrylic acid composition comprising hydroquinone monomethyl ether. 14. Hydroquinone monomethyl ether is a preoxygen active N-nitrosophenyl ether. 14. The composition of claim 13, wherein the amount is in molar excess relative to droxylamine. . 15. Pre-oxygen active N-nitrosophenylhydroxylamine is Hydroquinone monomethyl ether is present in an amount of 10 to 400 ppm based on the amount of acrylate. The composition according to claim 13, wherein 100 to 500 ppm based on the weight of the acid is present. thing.